There has conventionally been known an interactive robot that communicates with a user by providing messages by voices, characters, or the like. It is desirable that such an interactive robot provides an optimal response message in response to, for example, a question given by the user. There have been developed various techniques each allowing an interactive robot to optimally respond to a user. For example, Patent Literature 1 discloses a technique in which (i) an interactive robot catches a voice from a user and transmits the voice to a computer and (ii) the computer determines a responding sentence, which corresponds to the voice, to be outputted to the user.